1. Field of the Invention
The present invention relates to a power transmitting device for a vehicle for transmitting a driving force from a driving source such as an electric motor through a differential gear unit to right and left axles of the vehicle, and more particularly to a power transmitting device for a vehicle in which an electric motor for driving the axles, a planetary gear type speed reducing mechanism, and a differential gear unit are arranged in coaxial relationship with the axles.
2. Description of the Related Art
Proposed in U.S. Pat. No. 441,877 is a power transmitting device for a vehicle having a structure such that right and left axles of the vehicle are connected to a differential gear unit and a driving force from an electric motor coaxially provided on the outer circumference of one of the right and left axles is transmitted through a planetary gear type speed reducing mechanism to the differential gear unit. In this power transmitting device, the electric motor for driving the axles and the planetary gear type speed reducing mechanism for reducing the rotational speed of the electric motor are coaxially provided on the outer circumference of one of the axles, and all of the electric motor, the planetary gear type speed reducing mechanism, and the differential gear unit are accommodated in an integral housing.
The planetary gear type speed reducing mechanism has a sun gear connected to a rotor of the electric motor provided on the outer circumference of one of the axles, a ring gear fixed to the housing, a plurality of planetary gears each meshing with the sun gear and the ring gear, and a planetary carrier for carrying the planetary gears. The planetary carrier is connected to a differential case of the differential gear unit. Accordingly, a driving force from the electric motor is reduced in rotational speed at a set reduction ratio, and then input through the planetary carrier to the differential gear unit. Thereafter, the driving force is output from the differential gear unit to the right and left axles.
In this power transmitting device, both the electric motor and the planetary gear type speed reducing mechanism are coaxially provided on the outer circumference of one of the axles, so that the power transmitting device can be arranged compactly around the axles. Further, disclosed in Japanese Patent Laid-open No. 2001-121980 is a power transmitting device for a forklift having a structure such that an electric motor, a planetary gear type speed reducing mechanism, and a differential gear unit are coaxially provided on the outer circumference of one of the right and left axles and a driving force from the electric motor is transmitted to the right and left axles. A ring rear of the planetary gear type speed reducing mechanism is fixed to a housing, and a planetary carrier of the planetary gear type speed reducing mechanism is formed integrally with a differential case of the differential gear unit.
In these power transmitting devices described above, an oil reservoir intended for lubrication or the like is provided at the bottom of the housing. A dynamic level of oil stored in the oil reservoir is set below a radially outermost position of the rotor of the electric motor, so as to suppress oil agitation resistance by the planetary gear type speed reducing mechanism and/or oil agitation resistance by the rotor of the electric motor.
In the case that each pinion gear (planetary gear) of the planetary gear type speed reducing mechanism is provided by a dual-stage pinion gear or a double pinion gear composed of a large-diameter gear meshing with the sun gear and a small-diameter gear meshing with the ring gear, each large-diameter pinion gear is immersed into the oil stored in the oil reservoir during rotation, so that the oil is splashed up by each large-diameter pinion gear. In this case that each pinion gear is provided by such a dual-stage pinion gear, it is considered that the planetary carrier for carrying the dual-stage pinion gear is composed of a pair of annular members axially spaced apart from each other and a plurality of connecting portions for connecting these annular members in an integral manner.
In this case, each connecting portion is immersed into the oil during rotation of the planetary carrier, so that oil agitation resistance due to the oil splash-up by each connecting portion is generated. As a result, power transmission efficiency in driving and in energy regeneration is reduced to cause a reduction in fuel economy. In the case that each connecting portion is formed as an arcuate member for connecting the outer circumferences of the pair of annular members, the oil splashed up by each large-diameter pinion gear is hindered by each connecting portion during rotation of the planetary carrier, so that the oil cannot drop into the oil reservoir and the amount of oil in the oil reservoir is therefore reduced.
As a result, the oil level in the oil reservoir is lowered and an oil inlet of an oil strainer is therefore exposed to the atmosphere in turning, sudden starting, uphill running, or downhill running of the vehicle, causing the occurrence of aeration such that the air is mixed into the oil. The occurrence of such aeration causes a reduction in thrust force of a clutch in a hydraulic control system, resulting in difficulty of clutch control. Further, in the case that each connecting portion of the planetary carrier has an arcuate shape, torsional rigidity against a gear reaction can be easily ensured, but it is disadvantageous in bending rigidity in the radial direction. Accordingly, the wall thickness of each connecting portion must be increased in order to ensure a sufficient bending rigidity in the radial direction, causing an increase in weight of the planetary carrier.
In the prior art disclosed in the patent documents mentioned above, the ring gear of the planetary gear type speed reducing mechanism is fixed, so that the rotor of the electric motor is connected through the planetary gear type speed reducing mechanism to the differential gear unit so as to always provide a fixed speed reduction ratio. As a result, the rotor of the electric motor is always rotated according to the rotation of the axles during running of the vehicle. There is a case that the rotor of the electric motor may be forcibly rotated at high speeds according to the circumstances, causing an increase in loss in the electric motor and the production of undesired friction on the axles.